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Mclaren924
Joined: 13 Oct 2021 Posts: 199 Location: California
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Posted: Sat Aug 10, 2024 4:48 am Post subject: |
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Oh wow I grossly overestimated the price. Will certainly be doing head studs then if machine work is done, are they a direct fit or does the block need to be tapped for them? Besides that I just got done checking PTV with a degree wheel and some checking springs and this is what I found. .380 ptv for intake and .440 for exhaust. This is with a stock 924 bottom end and a stock 931 top end. Seems like there is plenty of PTV to take 100 of the deck. Would love if others can chime in on my measurements and tell me I'm way off or on track here lol. My next step is calculate the actual compression ratio I'd be dealing with if I took X amount off. Lot's of people talk about doing this but I see very little actual real world results and posts of what actually would result. I just got a quote from local well know speed shop for around 1300$ for valve job(no assembly), skim head, and deck block. WOW. Called another shop that's much less popular and they said 400 to do valves and skim and then waiting for a quote on block decking. That seems more like it lol. Just in theory... I wonder if you could take 200 of the block? I measured about .170 of a step between deck height and piston height at max throw. My concern is what happens if the piston starts to stick above the deck height? With my measurements you could really take off about .300 off the deck and leave the intake valve with .080 and exhaust with .140. All in theory lol. Yes you would have a very high CR that would most likely need ethanol to survive. Somebody tell me I'm crazy. _________________ 1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon |
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safe
Joined: 18 Mar 2017 Posts: 630 Location: Sweden
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Posted: Sat Aug 10, 2024 5:51 am Post subject: |
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Mclaren924 wrote: | Debating if the stock pistons will even be able to handle enough boost for the head to lift. |
I'm pretty sure they will. I suspect my head is lifting and will add the ARP head studs, when I've fixed my brakes. _________________ /Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Mclaren924
Joined: 13 Oct 2021 Posts: 199 Location: California
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Posted: Sun Aug 11, 2024 2:57 am Post subject: |
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For how cheap they are, only 80$ more or so than regular head bolts. It's a no brainer to me IF I have machine work done. Still wondering about my PTV measurements accuracy and how much I can really take off the block. Been playing around with Ideolas CR calculator and with 77 Na rotating assembly and 931 head I have 6.5-1 CR. Not ideal. .100 off the block gives me 7.5-1, mildly better but I'm chasing 10-1 or higher. .200 thou gives me roughly 9.1-1. This would be better and at least close enough of my goals to run. My concern with this much deck height removed is the quench area and what ill effects could I see from having the piston even or above deck height? Research more on that topic now but not seeing a ton of proven info. It may be worth hunting down some later 80 Na pistons to help bring compression up without needing so much deck. Oh the fun of the Rabbit Hole _________________ 1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon |
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safe
Joined: 18 Mar 2017 Posts: 630 Location: Sweden
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Posted: Sun Aug 11, 2024 4:04 am Post subject: |
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Mclaren924 wrote: | For how cheap they are, only 80$ more or so than regular head bolts. It's a no brainer to me IF I have machine work done. Still wondering about my PTV measurements accuracy and how much I can really take off the block. Been playing around with Ideolas CR calculator and with 77 Na rotating assembly and 931 head I have 6.5-1 CR. Not ideal. .100 off the block gives me 7.5-1, mildly better but I'm chasing 10-1 or higher. .200 thou gives me roughly 9.1-1. This would be better and at least close enough of my goals to run. My concern with this much deck height removed is the quench area and what ill effects could I see from having the piston even or above deck height? Research more on that topic now but not seeing a ton of proven info. It may be worth hunting down some later 80 Na pistons to help bring compression up without needing so much deck. Oh the fun of the Rabbit Hole |
I wouldn't bother with the turbo head unless you are chasing ultimate power levels (500+) but then you would have custom piston anyway. _________________ /Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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peterld
Joined: 10 Dec 2006 Posts: 955 Location: Noosa Heads QLD Australia
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Posted: Sun Aug 11, 2024 9:02 pm Post subject: |
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Confirm the ARP head studs are 251-4701 (Ford Cosworth Sierra/Escort).
Just run a tap down your block to chase the threads before installation (which should be done with everything). Important to follow ARP instructions on installation.
FWIW the crank and rods on all 924s are forged! As always, the pistons are the weak point, though NA versions are stronger in the ring land area. _________________ 80/81 932/8 ROW |
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Mclaren924
Joined: 13 Oct 2021 Posts: 199 Location: California
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Posted: Mon Aug 12, 2024 5:31 am Post subject: |
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I wouldn't bother with the turbo head unless you are chasing ultimate power levels (500+) but then you would have custom piston anyway.[/quote]
I think one of the major overlooked factors of these motors is the huge quench zone they have, which in turn helps create detonation. Essentially the tighter the quench, the less chance of detonation and better heat transfer. Which is why I'm interested in the 931 head, which in turn needs a decked block to bring up CR from 6-1. This head swap allows plenty of PTV while allowing lots of material off the deck. I read a lot of mike931's head discussion and from what I concluded, the 931 head flows not that much better than a 924 one when both are stock(correct me where I'm wrong here) i.e not worth the work if that's the sole purpose. I want to see what these things are capable of with stock assembly and just some machine work. Really interested what a stock piston can handle with high boost and CR but protected with an IC, EFI, and ethanol as well as a much shorter quench (.04 thou) Realistically it's time on this earth will be finite cranked up to the moon with all stock rotating assembly but I want to figure out if finite is 1 min, 1 week, 1 month, 1 year, or so on. I know I'm nuts, but if you are reading this chances are you're in the same looney bin as me lol. _________________ 1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon |
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safe
Joined: 18 Mar 2017 Posts: 630 Location: Sweden
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Posted: Mon Aug 12, 2024 5:46 am Post subject: |
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I'm not sure how much you will need to deck the block, but to really have any use of quench you need to get the piston to about 1 mm from the head, anything more is not quench.
But if you are using ethanol you will not have any detonation problem with the stock setup, I know I run 29 psi. _________________ /Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Cedric
Joined: 27 Aug 2004 Posts: 2674 Location: Sweden
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Posted: Mon Aug 12, 2024 5:53 am Post subject: |
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I think e85 is available in Cali? Just copy Magnus "safe" setup, very simple engine wise and more hp than whats really practical in a 924, at least thats how it felt behind the wheel with 420 or whatever hp it has at full boost _________________ 1980 924 Turbo
www.instagram.com/garagecedric/ |
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Mclaren924
Joined: 13 Oct 2021 Posts: 199 Location: California
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Posted: Mon Aug 12, 2024 11:17 am Post subject: |
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Safe or Magnus, I'm not sure what you prefer. Your turbo build and related threads are truly what inspired this build of mine, seeing what you've done on a stock 924 engine is nothing short of impressive. I have a completely stock with a dirty rebuild NA 9-1 motor that will be going in as the test mule for the chassis and efi/turbo setup. Basically a copy of your na setup. I will use that motor to flesh out and fully develop the EFI and turbo kit and get data and experience with these motors when tuning. I'm a whore and always want more though, I see the CR and improved quench as a means to get it cheaply. I very well could be wasting my time and money, but I see a potential for all the boost with a higher CR, a win win.
safe wrote: | I'm not sure how much you will need to deck the block, but to really have any use of quench you need to get the piston to about 1 mm from the head, anything more is not quench.
But if you are using ethanol you will not have any detonation problem with the stock setup, I know I run 29 psi. |
Precisely my idea here, there is no quench taking place on stock motors whether 924 or 931, .040 thousands is roughly 1mm, almost the same thickness as the compressed HG. The Piston will have to be roughly level with the deck with a normal thickness head gasket. A stock block should need roughly .250 removed in order to achieve a small enough quench. Leaving over 130 ptv for intake and 180 for exh with a 931 head installed. Shooting to make roughly the same hp at a lower boost number due to the higher CR, theoretically the quench should help protect the piston from knock as the boost gets higher. Ultimately I will try and squeeze every last drop out of it and see what I get, I already know it's going to be a blown up motor but like I said how long can we make it live on the edge for. I see cylinder head pressure probably being your biggest issue without head studs. 29 psi is pretty crazy on stock head bolts, very few motors out there taking that and continuing on. I wonder how far you can take it when you get those studs installed, is the clutch/fueling/turbo capable of pushing for more? Will you take it further? Like I said I'm a whore for more lol.
Cedric E-85 is readily available here, 5 gas stations in 5 miles radius have it. That being said it's very hard to find true e-85 from the pump, had a buddy burn the pistons out in his boosted mustang running what turned out to be like e35 on a strict e85 tune. Life lesson to run flex fuel sensor and tune or always test the fuel before running it. I think with my plans, ethanol is a must. I would like to see how it does on pump gas(American 91) too, I would imagine it could at least handle low boost with an intercooler even at higher CR. This is where the envelope gets pushed though as forged pistons give you much more leeway with fuel issues. The stock ones will see little room for error at 10-1 and boosted, especially with pump gas. _________________ 1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon |
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safe
Joined: 18 Mar 2017 Posts: 630 Location: Sweden
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Posted: Mon Aug 12, 2024 7:02 pm Post subject: |
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Mclaren924 wrote: | Safe or Magnus, I'm not sure what you prefer. Your turbo build and related threads are truly what inspired this build of mine, seeing what you've done on a stock 924 engine is nothing short of impressive. I have a completely stock with a dirty rebuild NA 9-1 motor that will be going in as the test mule for the chassis and efi/turbo setup. Basically a copy of your na setup. I will use that motor to flesh out and fully develop the EFI and turbo kit and get data and experience with these motors when tuning. I'm a whore and always want more though, I see the CR and improved quench as a means to get it cheaply. I very well could be wasting my time and money, but I see a potential for all the boost with a higher CR, a win win.
safe wrote: | I'm not sure how much you will need to deck the block, but to really have any use of quench you need to get the piston to about 1 mm from the head, anything more is not quench.
But if you are using ethanol you will not have any detonation problem with the stock setup, I know I run 29 psi. |
Precisely my idea here, there is no quench taking place on stock motors whether 924 or 931, .040 thousands is roughly 1mm, almost the same thickness as the compressed HG. The Piston will have to be roughly level with the deck with a normal thickness head gasket. A stock block should need roughly .250 removed in order to achieve a small enough quench. Leaving over 130 ptv for intake and 180 for exh with a 931 head installed. Shooting to make roughly the same hp at a lower boost number due to the higher CR, theoretically the quench should help protect the piston from knock as the boost gets higher. Ultimately I will try and squeeze every last drop out of it and see what I get, I already know it's going to be a blown up motor but like I said how long can we make it live on the edge for. I see cylinder head pressure probably being your biggest issue without head studs. 29 psi is pretty crazy on stock head bolts, very few motors out there taking that and continuing on. I wonder how far you can take it when you get those studs installed, is the clutch/fueling/turbo capable of pushing for more? Will you take it further? Like I said I'm a whore for more lol.
Cedric E-85 is readily available here, 5 gas stations in 5 miles radius have it. That being said it's very hard to find true e-85 from the pump, had a buddy burn the pistons out in his boosted mustang running what turned out to be like e35 on a strict e85 tune. Life lesson to run flex fuel sensor and tune or always test the fuel before running it. I think with my plans, ethanol is a must. I would like to see how it does on pump gas(American 91) too, I would imagine it could at least handle low boost with an intercooler even at higher CR. This is where the envelope gets pushed though as forged pistons give you much more leeway with fuel issues. The stock ones will see little room for error at 10-1 and boosted, especially with pump gas. |
Alright! Yes, .250 seems the right amount from what I remember. If that is doable it would be interesting to see what happens. You could probably push 400 with less boost that I'm using. You would need a significantly shorter timing belt and possibly there could be an issue with the tensioner getting very close to the cam gear.
Whats limiting my engine now is cylinder pressures and resulting overall torque. Clutch is reaching its limit.
If I could raise the rev limit and manage the torque (cylinder pressure) better, that would be good. Here I see the 931 head being a really good upgrade with some porting, because it responds very well to that.
If the rev limit could be raised to 7500 ish, the engine could be pushed further, but I think lighter rods and pistons are required for that. I'm planing to do this, but more imminent issues is the gearbox, the syncros was crap when I got the car and they haven't self healed... The torsion tube is rebuilt but the are known to snap at these torque levels.
I might replace the head studs one by one and see if I can save the stock head gasket.
Your plan is interesting, It would be comparing NA to Turbo heads with most everything else the same.
One thing that would be interesting is which head that is stronger, less flexy. You could probably drill and tap for even bigger head studs, if the head it self is strong enough. Many engines require more head studs and/or inserts into the head to keep it down and to not flex. I think the 924 heads are stiff with how tall they are.
Flexfuel sensor is a must. From what my tuner said, if you are above E60-E65 you are fine on detonation.
On Pump gas I ran 14-15 psi, 235 whp, limited by the clutch at 330 Nm. But also ignition limited. Just switching to E85 raised the power to 250-260 at same boost.
There is many things I want to do to push the engine further. But keeping the clutch/torsion tube/gearbox from exploding is first on the list.... _________________ /Magnus, Stockholm Sweden
=======================
Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Cedric
Joined: 27 Aug 2004 Posts: 2674 Location: Sweden
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Posted: Mon Aug 12, 2024 9:53 pm Post subject: |
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Squish will do stuff already from several mm height, but of course the effect is increased as the clearence are reduced. On slow burning engines with low air charge motion like ours it will definitely affect the combustion (it does of course on modern engines aswell), which will be seen in torque, combustion stability, EGT etc, could be even more important than CR sometimes, but there is a balance of course where to much can cause other issues like detonation. Without any tools on our hands the only way to find out what works best for the particular power goals and hard ware setup is to try it. Whats best for 0,7 bar non intercooled might not be the best for 2bar boost on E85.
Will be interesting to see where you end up, be sure to spend money on a very good turbo, well matched for the boost and flow targets, not all turbos are suited for low (relatively) flow and very high pressure ratios that are needed here. That and E85 is what enables these kind of silly numbers. _________________ 1980 924 Turbo
www.instagram.com/garagecedric/ |
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safe
Joined: 18 Mar 2017 Posts: 630 Location: Sweden
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Posted: Mon Aug 12, 2024 10:39 pm Post subject: |
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Cedric wrote: | Squish will do stuff already from several mm height, but of course the effect is increased as the clearence are reduced. On slow burning engines with low air charge motion like ours it will definitely affect the combustion (it does of course on modern engines aswell), which will be seen in torque, combustion stability, EGT etc, could be even more important than CR sometimes, but there is a balance of course where to much can cause other issues like detonation. Without any tools on our hands the only way to find out what works best for the particular power goals and hard ware setup is to try it. Whats best for 0,7 bar non intercooled might not be the best for 2bar boost on E85.
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David Vizard talks about heron heads in one of his books, can't find the section now, but what he wrote (from my memory) was that the closer you could get the piston edge to the head the better, mostly.
In another chapter about quench / squish he says the optimum is 0.03"-0.04" (~0.8-1 mm), the worst would be 0.1 to 0.125 (~2.5-3.2 mm) and would actually promote detonation. This was for a small block wedge head. _________________ /Magnus, Stockholm Sweden
=======================
Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Mclaren924
Joined: 13 Oct 2021 Posts: 199 Location: California
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Posted: Wed Aug 14, 2024 3:53 am Post subject: |
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The bigger headstuds may be needed but that won't be until far down the road when aftermarket rotating assembly comes out to play. The trans is our serious limiting factor I feel as they were simply never dreamed of holding so much power and then combined with the transaxle design, are susceptible to breaking from rough driving or too much power and or a mix of both. After visiting Britain and his graveyard of 13 blown up g31, there may be more; I followed his course and currently have a corvette 6 speed and torque tube sitting in the corner waiting to get TT bearings fixed and then it can be grafted into my car. Much easier said than done but way way way and i mean way cheaper than a 968 6 speed. Also probably better suited for rowdy driving and torque/hp. That project will probably wait until I run out of porsche style transmissions or reach the point they just cant survive. Street and Drag strip launches will not be easy on these trans of ours so I full expect that to be a huge issue I face.
The improved quench is really a biproduct of me chasing higher CR and the "theoretical ideal" quench would be perfectly the same as where I want my CR to sit. One could say a happy accident lol. Silly numbers are the goal here but at the end I'm not too worried with HP results and are looking for actual useable track/street results for my answers. I've seen and even helped build a few crazy number motors that were a nightmare to get drivable. As you said Cedric the turbo choice is imperative here, something big enough to blow the top off the place but spool fast enough that I don't feel like I'm driving a dial up turbo. It is interesting seeing most high psi turbos are also high flow turbos, great point to match the flow rate. Truly very excited to see what this motor can yield. _________________ 1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 "Pepper" Restoration (almost done)
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon |
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Beartooth
Joined: 05 Apr 2022 Posts: 244 Location: Roberts, MT
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Posted: Wed Aug 14, 2024 4:37 pm Post subject: |
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I think if you're after quench, a turbo head with a custom piston might be necessary. I dug through the available off-the-shelf forged pistons combined with either Mitsubishi or custom rods, and couldn't come up with anything that compelling. With the Mitsu rods and (ironically) Mitsu pistons, you'd be looking at 2.8mm deck clearance for 9:1 compression - not good quench range. By offset grinding the rod journals, you'd gain a bit of displacement and reduce the amount of decking needed (maybe enough to still use a standard timing belt). You could be in ideal territory for a NA motor, netting about 10.5:1 compression with 1mm deck clearance. But that also takes decking the block almost three MM. Moving on to a custom rod, you could then use a Mahle piston intended for the BMW B32, which has a bit more dish and would give you 9.3:1 compression with 1mm deck clearance - only that piston has a four-valve relief that would hamper quench characteristics.
Unfortunately, you just have a ton of compromises trying to make off-the-shelf pistons work without losing quench: there just isn't a mass-produced piston with enough dish to work with such a small head, nor with such a tall compression height. I've been leaning toward custom pistons when I rebuild my 931, but I don't really want to know what those would cost. So I haven't looked into that yet; I'm working feverishly on finishing up my shop of late anyway. So it depends on one's budget. With cost as no object, I'd think a custom billet crankshaft and custom pistons (which would allow you basically pick and choose a rod) would be what you'd want. Set the quench as tight as you dare (I've heard 1mm - .040" - works well and should be safe), pick a dish around what compression you want to run, and you're done. If you've got a wad of cash, of course. A custom piston alone of course allows you whatever compression and quench you want too - the downside being the heavy rods, and you could get more stroke with a custom crankshaft.
So I'm not sure what to recommend, or I'll end up doing. I'm not planning on pushing the envelope as much, so I might use S2 pistons if I can find a good set (hoping that knock sensing, an intercooler, and careful tuning will still allow a bit more boost than stock). But I might be stuck reverting to flat-tops. One interesting option there is I see Wiseco makes a 4032 forged flat-top for the Chrysler six-cylinder. https://www.summitracing.com/parts/wis-pts536a45 It's a little too much compression for what I have in mind, unfortunately, but maybe not with E85. I wonder if it'd be possible to mill just a bit off to drop compression though... It looks like similar option to the VW 87MM pistons, but on those, I can't seem to find anywhere sells just the piston, and I can't find any information on it from Mahle themselves, so I don't know.
On the transmission, I think the bigger quad-cam Audi motor and demands of racing are most of what's driving the need for more transmission in Britain's case. With a working G31, I'd spend my money elsewhere first, although I wouldn't risk one that worked perfectly (whether original or rebuilt). If you're really dedicated to pushing to the limit, I suppose it makes sense to use the Corvette transaxle. I would be curious to see - short of running race or near-racing tires - whether the engine or transmission (G31) would go first in a build like this though. Isn't safe's car running a G31? _________________ 1980 931 diamond in the rough |
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Cedric
Joined: 27 Aug 2004 Posts: 2674 Location: Sweden
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Posted: Wed Aug 14, 2024 7:33 pm Post subject: |
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safe wrote: | Cedric wrote: | Squish will do stuff already from several mm height, but of course the effect is increased as the clearence are reduced. On slow burning engines with low air charge motion like ours it will definitely affect the combustion (it does of course on modern engines aswell), which will be seen in torque, combustion stability, EGT etc, could be even more important than CR sometimes, but there is a balance of course where to much can cause other issues like detonation. Without any tools on our hands the only way to find out what works best for the particular power goals and hard ware setup is to try it. Whats best for 0,7 bar non intercooled might not be the best for 2bar boost on E85.
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David Vizard talks about heron heads in one of his books, can't find the section now, but what he wrote (from my memory) was that the closer you could get the piston edge to the head the better, mostly.
In another chapter about quench / squish he says the optimum is 0.03"-0.04" (~0.8-1 mm), the worst would be 0.1 to 0.125 (~2.5-3.2 mm) and would actually promote detonation. This was for a small block wedge head. |
What works in one combustion chamber/port/valves doesnt necessarily translate another, with 2 bar of boost, different fuel etc, only way to find out is to do something reasonable and try _________________ 1980 924 Turbo
www.instagram.com/garagecedric/ |
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